A fundamental concern in the petroleum products industry relates to underground storage of hazardous fluids in large tanks. As a large tank is installed, it is normally filled first with gasoline, or whatever fluid is intended for its use, before earth is back filled and compacted around the tank so that settling is minimized. The added weight of the liquid assists in locating and anchoring the tank.
Over time, as the tank is emptied and refilled, the buoyancy of the tank changes. Rather than remain in one place as might be expected for a large tank packed into the ground, it has been discovered that the tank can shift. If the surrounding soil is wet or subjected to underground water table pressure, the tank may "float" underground. Even when the tank is prevented from substantial movement, such as when the ground above the tank has been filled and covered with concrete or asphalt, the tank and attachments thereto are subjected to considerable force. This phenomenon is known as "shifting tank syndrome". As a result of the "shifting tank syndrome", pipes, pumps, sumps and risers and all of the various fittings and connections associated with the tank and with the pipe system are subjected to substantial stress. With these stresses comes the risk of failure of the tank and the attachments, especially failure of the joints and connections.
Because the petroleum equipment market is becoming more environmentally conscious, a shift has occurred from the use of rigid underground piping to flexible piping. Rigid piping has a number of drawbacks, including the fact that it must be laid out accurately with precise angles using 90.degree. elbows, 45.degree. elbows and the like, even when the underground fuel storage tanks and pumps are not properly aligned with respect to each other.
Flexible underground piping systems have also gained popularity because these systems are more easily installed than rigid piping systems. Rigid systems require on-site measurements and cutting of each piece, along with the intensive labor involved in making two pipe connections every time the piping undergoes a change in direction. Flexible underground piping systems thus can be installed at a lower total cost than conventional rigid piping systems.
As part of the development of underground piping systems, it has been necessary to provide a means of secondary containment for both the primary piping and for the associated fittings in order to provide a margin of safety in case of leaks or damage to the system. The secondary containment pipe protects the primary supply pipe from the ambient environment and from inadvertent damage, and further, provides for a method of containing the fluid from the primary pipe in the event of a rupture or leak of the primary pipe.
Several types of double wall flexible piping systems are known. These include systems which have a small flexible pipe housed in a larger flexible containment pipe. One such system is described in U.S. Pat. No. 4,971,477 (the '477 Patent), wherein access chambers are separated by a secondary containment pipe which is sized to accept a primary pipe within the secondary containment pipe. This system, however, has a number of drawbacks, including the fact that the primary and secondary pipes are installed separately and that fittings are required for each of the two types of pipes. Moreover, because each of the pipes connects with an access chamber, a rapid and simple method of monitoring the condition of the primary piping may not be possible. Although the sumps in the '477 Patent are capable of being monitored visually by an attendant, but the demand of other tasks to be performed by the attendant eventually leads to fewer and fewer inspections. Most important is that leaks may occur randomly, not only just prior to a visual inspection, and especially when inspections grow less frequent over time.
As taught in the '477 Patent, the flexible piping and the secondary containment piping require a sump or access chamber each time that sections of the flexible pipe are joined together. Most other systems also require a sump at each location where sections of flexible pipe are joined together.
Other systems which have met with some degree of success using coaxial pipes and a secondary containment pipe are shown in U.S. Pat. No. 5,263,794 (the '794 Patent), the entire disclosure of which is incorporated herein by reference.
The '794 and Patent disclose various definitions that have become standard in the industry, for example, "tanks", "pumps", "dispensers" and the like. Also disclosed therein are descriptions of double wall piping systems which provide secondary containment. In a system that employs secondary containment, a primary pipe carries the petroleum product or other hazardous material from the underground tank to the above-ground dispenser. The primary pipe, also known as the supply pipe, is located inside a larger, outer secondary containment pipe, known also as the containment pipe. Access sumps and other containment components are located around the tank pump, underneath the dispenser and at various junctions of piping.
Various types of double wall piping systems are discussed in my U.S. Pat. No. 5,297,896. Among those systems are: (1) a non-flexible fiberglass supply pipe fully contained by a larger non-flexible fiberglass containment piping system; (2) a non-flexible fiberglass or steel supply pipe contained by a combination of larger, flexible and non-flexible polyethylene telescoping containment pipe; (3) a system like the preceding one but with a larger, non-flexible polyethylene telescoping containment pipe; (4) a fiberglass or steel non-flexible supply pipe contained by a flexible membrane trench liner; (5) a system like the preceding one but with a non-flexible fiberglass trench liner; (6) a flexible nylon composite supply pipe contained by a larger flexible polyethylene containment pipe; and (7) a flexible rubber composite supply pipe contained in a larger, flexible polyethylene composite containment pipe.
Some or all of these pipe systems have been utilized. More recently, a significantly more effective system has become available and has met with substantial success in the industry. This more effective pipe system's supply pipe is a flexible double wall pipe comprising an inner pipe and an outer pipe in radial communication with the outside surface of the inner pipe. Most preferred are pipes of this configuration that have internally facing longitudinal ribs on the inner surface of the outer pipe, or externally facing longitudinal ribs on the outer surface of the inner pipe. In either such design, a plurality of circumferentially spaced ribs extend radially from one pipe member to the other pipe member such that the ribs have a surface that confronts and snugly engages the other pipe to define an interstitial space between the two pipes.
The flexible double wall pipe described immediately above is disclosed in my U.S. Pat. Nos. 5,297,896 and 5,527,130, entitled Environmentally Safe Underground Piping System, the entire disclosures of which are incorporated herein by reference. The coaxial pipes disclosed in the these patents are normally suited for use with hazardous material transfer pipe systems of the type described herein. The inner most layer is formed from a material such as nylon, polyethylene or the like, which is highly resistant to the hazardous transfer fluid. The outer jacket of the double wall pipe which is exposed to the ambient underground environment is formed from a material such as nylon, polyethylene or the like, which is highly resistant to the ambient underground environment and which does not degrade over time. Between the outer wall of the primary pipe and the inner wall of the containment pipe is an intermediate layer, either in the form of ribs projecting from one surface to the other, or in a standoff layer formed from a cylindrical portion having circumferentially spaced ribs that define the interstitial space between the two pipes. As noted in the '130 Patent, other layers can be present in the design, such as intermediate layers formed from lower cost materials which do not directly contact either the hazardous material being transported or the ambient underground environment.
The environment for both surfaces of the pipe is an important design aspect which needs to be considered. Product components which make up primary or secondary containment systems for hazardous liquids, and in underground applications particularly, must be designed, manufactured and individually tested to insure that they will not fail due to material deterioration. The selection of plastic material used in these components must be capable of withstanding long term exposure to a variety of conditions expected in this type of underground system. Some of the most common conditions to which the pipe would be exposed are contact with petroleum fuels, alcohol blended fuels, brown water, microbial growth, high humidity and heat. For example, if the wrong plastic were selected, the component could fail due to degradation and the contained hazardous liquids could then escape into the environment. For example, the use of primary and secondary containment products made of exposed polyethylene has resulted in failures across the United States with reports of leaking products escaping into the surrounding environment. Some plastics, such as polyethylene and nylon, however, are excellent performers upon exposure to a wide variety of chemicals and conditions, and do not chemically degrade when exposed to these chemicals and conditions. The appropriate plastic material, such as nylon or polyethylene, does not have an adverse reaction to water, micro-organisms, solvents, heat, oxygen, sunlight or burning. Products designed for primary and secondary containment of liquid fuels in underground storage and piping applications may expect to be subjected to at least four of these seven conditions at any given time.
The material employed must be resistant to hydrolysis, as it is expected that water and high moisture conditions will exist in underground burial applications both for contained and non-contained underground piping applications. Resistance to hydrolysis is important since some elastomers suffer an irreversible breakdown when exposed for lengthy periods to hot water, moisture vapor or tropical climates. Resistance of the materials to solvents such as alcohols found in oxygenated fuels and many fuel additives is also important since solvents and fuel additives can be expected to be present in both contained and non-contained underground piping applications. Tests are available to assure that the plastic materials chosen will resist whatever hazardous material, such as a 90% fuel/10% methanol mixture that is being transported. Exposure to micro-organisms also must be expected in both contained and non-contained piping applications because of the existence of high moisture and heat. Temperatures inside tank sumps have been found to exceed 100.degree. F. due to solar 15 heating of the steel manhole covers located just above the tank sumps. In this type of environment, micro-organisms multiply very rapidly and the enzymes released by these micro-organisms can cause breakdown of certain chemical linkages and destruction of some forms of elastomers. Care therefore should be taken to select materials which are nondegradable when contacted by the specific environment in which they are employed.
In addition to the tanks, pumps, pipes and dispensers used in underground piping systems, sumps are used as part of the secondary containment system. One sump surrounds the pump on the tank and another sump is generally positioned below each dispensing system. Even with the use of flexible piping, a complex underground piping system involves placement of many sumps and other fixtures, all of which are subject to attack by the ambient environment, transfer fluid and to stresses caused by the shifting tank syndrome.
As can readily be appreciated, every pipe must eventually end. It then becomes necessary to connect that end of the pipe to either a pump or to a fitting joining two or more other pipes. Typically the inner, primary supply pipe, is directly connected to fittings and the like, and another primary supply pipe is connected to the other end or ends of the fittings. Merely connecting the inner supply pipes, as in the past, has been no different than connecting a single pipe system. In the evolution of piping systems as discussed above, the relationship of the outer, secondary containment pipe to the system has become more complex.
Initially, non-flexible pipes functioning as a secondary containment pipe were attached to the sump or other chambers by enlarging the hole in the sump to permit entry or exit of the outer pipe from the sump, and later by various fastening and sealing methods and devices. Initially, the interstitial space between the inner and outer pipes served as a conduit for fluid leakage to flow downhill into the next sump in the piping system. Leaks could come from fuel from the inner pipe, or from the outer pipe as ambient environmental liquids, such as water, penetrate the outer pipe.
All of the secondarily contained piping systems, access sumps and other equipment described above have been developed over a short period of time in response to a continuously changing industry where environmental and safety regulations are becoming more strict. As these developments occur, various manufacturers and other organizations have developed their own design criteria in response to the concerns which they have. Accordingly, none of the systems described above is completely effective in resolving the environmental issues while maintaining an efficient and effective fluid transfer piping system.
There are several additional considerations that need to be addressed in designing of the connections between sections of pipes, particularly between sections of flexible coaxial pipes. First, it is desirable to avoid plastic to plastic connections, since vibration and time will cause flow of the plastic material so that an effective connection is not always secured. A much better seal is achieved when metal and plastic are joined together since the metal does not melt or flow and the plastic tends to accommodate itself to the metal. However, in such cases it is necessary to protect the metal from the environment by a coating to avoid a corrosive and unsafe condition. Coatings on metal parts, however, often peel off or become damaged during handling of the metal parts. Also, if the coatings are too thick, the necessary metal to plastic contact to establish an effective seal is not achieved.
As will be apparent from reviewing the above patents, there is an interstitial space between the inner primary supply pipe and the outer secondary containment pipe. This interstitial space has been used to transfer leaked fluid into the containment sump or access chamber. Typically, in early systems, the access chambers were inspected on a regular basis to see if quantities of fluid had collected. This, of course, does not provide a rapid response to a major leak of fluid such as fuel from the primary supply pipe.
As shown in the '794 and '876 Patents, the entire system is connected such that the primary pipe functions as a closed system, transporting fuel from the supply tank to the various dispensers. However, the interstitial space between the primary supply pipe and the secondary containment pipe is, at least in '477 Patent, merely a conduit allowing leaked fluid to flow to an access chamber for observation. Although it is possible to monitor the conditions of one or all of the access chambers, for example by visual inspection, no simple method of monitoring the entire system is possible.
Accordingly, a principal object of the present invention is to provide an underground piping system which employs coaxial pipe, such as those pipes described in my '896 and '130 Patents, which include a primary supply pipe and a secondary containment pipe, in which the interstitial space between the two pipes can be connected to the interstitial spaces in other segments of piping to form a continuous system of interstitial space.
Another object of the present invention is to provide a coupling assembly for use with coaxial pipes which permits coupling of the interstitial space between the coaxial pipes with corresponding interstitial spaces in other segments of pipe.
Still another object of this invention is to provide a coupling assembly which facilitates connection of a plurality of coaxial pipes without the use of containment chambers or sumps.
Yet another object of this invention is to provide a coupling assembly in which sealing between the pipes is accomplished by a plastic on metal seal in a manner that protects the metal portion of the seal from exposure to the environment so that the coupling assembly will operate effectively underground without additional protection from sumps and the like.
An additional object of this invention is to provide an effective monitoring system utilizing the interconnected interstitial space of the coupling assembly, particularly with the use of vacuum applied to the interstitial space.
Finally, it is an object of this invention to provide an underground piping system that can be directly buried in the ground without the need for sump devices at every junction of pipes or fittings and without the need for separate secondary containment of any type.
Other objects will appear hereinafter.